The present invention is concerned with remote control systems and is particularly applicable to such systems in which spurious signals are likely to occur, as when the system uses an acoustic signal link of sonic or ultrasonic frequency.
An ultrasonic control system could be based upon pulse code modulation or a similar coding system but then becomes very sensitive to spurious signals and sigdistortions caused by reflections and multiple path propagation. Conventionally, therefore an ultrasonic link is normally based upon multi-frequency coding, even though this requires accurate frequency control and can suffer from errors when either the transmitter or receiver of the link is moving.
The present invention therefore proposes a pulse code modulation system with provision for compensating for or at least detecting spurious signals and signal distortion.